The overall goal of the proposed research is an elucidation of some of the mechanisms of cellular and humoral immunity and tolerance to avian leukosis virus (ALV), ALV antigens, and virus-induced antigens. Several general objectives are envisaged. One goal is to determine whether congenital infection with avian leukosis viruses truly leads to a state of immunological tolerance to both virion antigens and virus- induced antigens in the chicken. The isolation and purification of Rous virus-induced surface antigen (VISA), the determination of its relationship to viral structural antigens and its role in various immune phenomena are integral parts of this study. Further, it is intended to determine the effects of congenitally transmitted ALV on cellular and humoral immunologic responses and biologic responses to other viruses of the avian leukosis-sarcoma complex.